1. Field of the Invention
The present invention relates to an image forming apparatus which is used as it is with one or a plurality of attachment units attached as needed.
2. Description of the Related Art
An image forming apparatus is already available which is used with one or a plurality of attachment units, such as multiple of sheet feed units or finisher units, attached in accordance with specifications or as desired by a user. Such an attachment unit usually comprises controlled elements such as various types of sensors which detect an operating state of the attachment unit and actuators which drive movable portions. Hence, it is necessary to lay communications lines which control these controlled elements between a main section of apparatus and the respective attachment units.
In the event that a plurality of attachment units are attached, it is necessary to control these units individually. While a solution may be to connect the plurality of attachment units respectively with the main section of apparatus by individual communications lines, this solution requires many communications lines and is costly. Hence, a general alternative is to connect the attachment units to each other in a cascade arrangement. Meanwhile, for further reduction of the number of the communications lines, a conventional apparatus and a conventional communications method also have been proposed which require to connect the respective attachment units and the main section of apparatus by a common bus line to thereby realize transmission of various types of data with time-division system in a serial communication. This type of apparatus is the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2001-106352 for instance. According to this type of technique, a communications module and decode/encode means such as a serial/parallel convertor are disposed to each attachment unit and serial data transmitted on a communications line is converted into a control signal which is for controlling each sensor, etc.
Such a conventional technique is effective particularly in a circumstance that there are many units are attached, since the number of communications lines does not increase even when a large number of units are used. However, a communications module and the like to be disposed to each unit demand relatively expensive components, and hence, there arises a problem that an apparatus cost also increases. In addition, since many pieces of data are transmitted in a time-division serial transmission, the process lacks the speed and is not very much resistant against a noise which gets mixed in the communications lines.
As a demand for a higher speed in the process has rose over the recent years, a communications technique is desired which makes it possible to stably control a plurality of attachment units at a high speed and to structure the attachment units at a low cost.